It is now well known that ultrasonic echoscopy techniques can be used to provide information about an object that is not visible to the eye. The basic technique of ultrasonic echoscopy involves directing a short pulse of ultrasonic energy, typically in the frequency range from 1 MHz to 30 MHz, into the region of the object that is being examined, and observing the energy that is reflected, as an echo, from each acoustic impedance discontinuity in that region. Each echo received is converted into an electrical signal and displayed as either a blip or an intensified spot on a single trace of a cathode ray tube or television screen. Such a display of the echoes is known as an "A-mode" echograph or echogram, and is useful in a number of diagnostic techniques to locate the boundaries of the object or to provide other information about the region into which the pulse of ultrasonic energy has been directed.
If a series of adjacent A-mode displays are obtained (for example, by physically or electrically moving the transmitting transducer which produces the pulses of ultrasonic energy, or by scanning the direction of transmission of the pulses of ultrasonic energy), a two-dimensional image of the object under examination may be displayed on the cathode ray tube or television screen. Such an image or display of acoustic discontinuities, which corresponds to the structure of the object, is known as a "B-mode" image or display.
The use of the Doppler frequency shift in the ultrasonic examination of flowing liquids and moving objects is also well known. Many echoscopes which perform the B-mode imaging examination described above can also perform Doppler frequency shift measurements in respect of echoes returned from moving objects within the region receiving ultrasonic energy from the echoscope. When the object under examination is a blood vessel, measurement of the Doppler shift of echoes from the blood cells within the vessel permits the velocity of those blood cells to be estimated. As pointed out by R W Gill, in his article entitled "Measurement of Blood Flow by Ultrasound: Accuracy and Sources of Error", which was published in Ultrasound in Medicine and Biology, Volume 11 (1985), pages 625 to 641, it is possible to measure the total volume of flow per unit time using an ultrasonic examination technique which includes the measurement of frequency changes due to the Doppler effect.
In ultrasonic examinations including Doppler frequency shift measurements, it is necessary to obtain echoes from a limited volume of the flowing liquid which is within the vessel being examined. This is achieved by fixing the line of sight of the ultrasonic transducer and, in the most commonly used version of Doppler measurement known as "pulsed Doppler", analyzing the echoes obtained from the sample volume for a limited range of time delays. The Doppler shift in the received echoes is averaged in order to calculate the average speed of scatterers in the flowing liquid.
In current implementations of the pulsed Doppler technique, the quantity measured as "velocity" is actually the component of velocity measured along the line of sight of a beam of ultrasound. Therefore the actual velocity (magnitude and direction) of the flowing liquid is not determined, although it can sometimes be inferred (for example, when the flow is along a vessel with clearly-imaged walls, as in the case of blood flow through an artery).
The information obtained by applying the Doppler technique to ultrasound measurements is commonly displayed in one of two ways or modes.
The first mode, known as a spectral display, is used when ultrasound pulses are repeatedly transmitted down the same line of sight, and echoes from a small region (the "sample volume") are selected for analysis. This selection is effected by accepting only echoes received within a certain range of delay times after transmission of the ultrasound pulses. The frequency spectrum observed, over a series of transmitted pulses, as a result of mixing the returned echoes with the transmitted frequency, corresponds to the spectrum of velocities in the flowing liquid that is being examined. This spectrum is displayed in a two-dimensional form with a horizontal axis representing time and a vertical axis representing velocity, and with the brightness of the displayed data corresponding to the strength of Doppler signal (which is approximately proportional to the number of scatterers in the flowing liquid at that time moving with the indicated velocity). Information about the direction and distribution of flow velocities, as well as their time evolution, may be inferred from this kind of display. The physical principles involved in this form of velocity display are well explained by K J W Taylor, P N Burns and P N T Wells in their book entitled "Clinical Applications of Doppler Ultrasound", published by Raven Press (1988).
The other commonly employed display mode is usually termed "color Doppler imaging". This display mode incorporates Doppler frequency shift information into a conventional ultrasound image. In this display mode, selected ultrasound lines of sight are repeated several times in rapid succession. Doppler shift measurements are taken for a number of sample volumes down each of the selected lines of sight. By suitable arrangement of the selected lines of sight, the whole or a part of the imaged area can be covered with a grid of sample volumes. A simplified version of the Doppler spectrum is calculated, and the liquid velocity is displayed by coloring the area of the ultrasound image which is covered by the sample volume. The color indicates the direction (towards or away from the transducer) and approximate magnitude of the velocity. In some applications of the color Doppler imaging technique, the spread of velocities can also be displayed. The display is updated in real time and gives an overview of liquid dynamics over an extended region. This technique has been described by K Miyatake, M Okamoto, N Kinoshita, S Izumi, M Owa, S Takao, H Sakakibara and Y Nimura in their article entitled "Clinical applications of a new type of real-time two-dimensional flow imaging system", which was published in American Journal of Cardiology, volume 54 (1984), pages 857-868.
In both the spectral and color Doppler imaging display techniques, the displayed velocity is actually the component along the line of sight, as indicated in the well-known Doppler equation: ##EQU1## in which f.sub.D is the Doppler frequency shift, f.sub.0 is the ultrasound frequency, v is the velocity of the liquid, c is the speed of sound in the medium and .theta. is the angle between the flow vector and the ultrasonic line of sight. Thus changes in the Doppler frequency shift f.sub.D may be due to changes in the liquid velocity v, or they may be due to changes in the angle .theta.. In some cases (for example, when there is undisturbed flow along a straight vessel), the angle .theta. may be inferred from the vessel orientation. A technique for doing this automatically is described by L S Wilson, M J Dadd and R W Gill in the specification of International patent application No PCT/AU91/00026. However, in many instances where liquid flow is being investigated by applying Doppler frequency shift measurements to ultrasound, the absolute magnitude and direction of the flow velocity cannot be easily inferred from the value of one velocity component.
Clearly it is desirable to be able to measure two orthogonal components of the velocity of a flowing liquid, to enable its velocity vector to be determined, and several alternative methods of doing this have been proposed. For example, M D Fox, in an article entitled "Multiple crossed beam ultrasound Doppler velocimetryle", published in IEEE Transactions on Sonics and Ultrasonics, volume SU-25 (1978), pages 281-286, describes the use of several transducers viewing the region of flow from different directions to obtain several velocity components. In addition, G E Trahey, S M Hubbard and O T von Ramm, in their article entitled "Angle independent ultrasonic blood flow detection by frame-to-frame correlation of B-mode images", published in Ultrasonics, volume 26 (1986), pages 271-276, describe the use of two-dimensional cross-correlation between successive B-mode image frames to determine the blood velocity vector. However, the first approach (described by Fox) uses a non-standard transducer arrangement, while the second approach (described by Trahey et al) requires larger computing power, making real time implementation of the technique difficult.